Passive Solar

  Why Solar?

The heavy cement structure (and possibly the surrounding earth) of an earth sheltered home provides a way to store a lot of heat energy (heat capacity), and modern insulation materials allow us to keep most of that heat where we want it, but we still need a heat source.  You could pay for that heat energy in the form of fossil fuels, wood or electricity (which can come from a variety of sources), as people usually do above ground.   However, it just so happens that sunlight is also full of thermal energy and its free (once the system is setup).   The combination of free energy and the capacity to store it is so compelling that “passive solar” energy is almost always used with “earth sheltering” in cold climates.  There are some exceptions to this, including homes by Malcolm Wells and Andy Davis, but all were more than a few decades ago and would probably be done differently if their builders had another chance.

Solar heating can be further broken down into active and passive systems, but even the active systems start with passive components, so we will start there also.

There are also ways to use passive solar for cooling and we will get to that.

Passive Solar Overview

Some define passive solar as a heating system with no moving parts, but that is not quite correct…   It actually has one really big moving part, the whole planet Earth.  It is the 23º28’ tilt of the planet, combined with its  orbit around the sun that makes “passive solar” possible.

 

In simplest terms, passive solar is designing and orienting the home and its windows & shades to let in, absorb and contain the energy of the sun in winter, but exclude the energy of the sun in summer.

Winter (the cold season) occurs because the sunlight comes in at a lower angle.  The Sun puts out a steady stream of energy in all directions all year and the earths orbit actually takes us a little closer to the sun in winter (northern hemisphere).  However, it is colder in winter because at the lower angle, that same amount of solar energy is spread over a larger area of the planets surface.  This is technically referred to as “the uneven distribution of insolation”.     Conversely, we have summer because the sun comes in at a higher angle and its energy is concentrated in a relatively smaller area (more energy per square ft or meter).  In addition to a higher angle, the summer sun actually rises north of east and sets north of west, where as the winter sun spends all of its time in the southern sky (from the perspective of the northern hemisphere).  Even though the winter sun is weaker (the energy is diluted) due to this lower angle, we can adjust the geometry of our homes to let in only the lower-angle southern sun and filter out the higher angle sun.   This can be done by increasing the percentage of glazing on the southern side of our homes and by extending overhangs or shades over the windows.

On a basic level, most architects are familiar with the concept and will automatically design some elements into your home based on rules of thumb such as  limiting the glazing to a certain percentage of the floor area to prevent overheating.   However, they may be less familiar with the theories behind the “rules of thumb”  (to balance the solar energy collected with the storage mass of a typical home) and may not be able to adjust them for different building materials.

In many cases, the home may be well designed, but then built with a different orientation than the architect had in mind…  Or perhaps the orientation is correct, but solar access is blocked by other homes, trees or landscape.

Practical Passive Solar

There are three main ingredients for a successful passive solar design; 1) A way for the solar energy to enter, 2) a way to store that energy for later use when the sun is not shining, 3) insulation and other measures that improve the efficiency of the system by reducing the amount of heat lost.

In practical terms, the first step is to site the home correctly such that solar gain is possible, at least in winter.  Even the ancient Greeks knew about this and considered solar access to be a right.  Ancient building codes prohibited blocking the sunlight of another citizen.  Greeks and Romans considered anyone who did not orient their home with respect to the sun to be an ignorant barbarian.  In modern times, height and setback restrictions aim to do the same thing, but you should not ignore the potential for someone to build on the lot south of you…  or perhaps trees are growing that could eventually pose a problem.   If you don’t own the view all the way to the horizon, plan accordingly.

Once an acceptable building site is found, placing most of the windows on the south side of the home will increase the amount of winter solar energy collected for a given amount of glass.  The invention of glass dramatically improved the effectiveness of passive solar because the higher frequency sunlight could enter, but the cold wind could not.  Also, after warming the interior surfaces, the lower frequency infra red heat waves could not exit easily thru the glass.  Modern glass, with its low E coatings, is even more able to trap those low-frequency infra-red waves in order to keep the free solar heat in the home.   Other improvements in insulation and reduction of infiltration also help retain heat and make passive solar more viable.  However, windows still have a much lower R value than modern walls, so reducing the number of skylights, north, east and west facing windows can prevent significant heat loss in winter.  The net effectiveness of passive solar is increased if the energy entering the southern windows is not immediately lost again.  Many passive solar experts also recommend insulating the southern windows when the sun is not shining.  Insulated curtains or even shutters can dramatically reduce the nightly heat loss.

Reducing the number of windows on the east and west side of the home (as well as the number of skylights) can help prevent overheating in summer.  Modern, above-ground passive solar designers recommend that you limit your glazing (windows) to prevent excessive sunlight from overwhelming your homes storage ability and overheating in the winter.  Due to this imbalance, PassiveHaus owners have seen plastic toys melt in their living rooms in winter.  Earth-sheltered homes, with their much greater mass, are much less susceptible to overheating and can glaze a larger percentage of their southern walls.

One way to increase southern windows and reduce east and west windows is to stretch out your home layout so it has a proportionally longer southern side. There are practical limits to this concept as hallway area is proportional to length and costs are proportional to the surface area, both of which grow with aspect ratio.     A theoretical optimum is to have a home that is one room deep with a hallway behind it.  Some homes also include a row of closets on the north side of the hallway to further insulate the living areas (always be careful to ventilate closets against a “cold” wall to avoid dampness and mold).  Others (including myself) compromise a little more and put utility rooms, storage rooms and even bathrooms on the north side of the hall.

A passive solar home will overheat in summer if steps are not taken to block high angle summer sun.   This can be done with overhangs or louvers or other attachments to the home that will not also block the lower angle winter sunlight.  These overhangs should be constructed to shade most of the window during most of the time between the Spring and Fall Equinox.

 

There is some debate about how far any overhangs should protrude.   A novice may extend southern overhangs to prevent sunlight from entering the windows when the sunshine is above the Equinox noon angle (90º minus N.Latitude of the home).   While this does effectively exclude summer heat, it may not let in enough winter sun.   During the months before the Spring Equinox or  after the Fall Equinox, the solar angle is still similar to the Equinox angle and only the bottom portion of the window may be receiving sunlight.   Solutions to this problem include adjustable awnings or compromising with a steeper angle that lets in some summer sun.   My personal favorite solution is a plant covered arbor…  In Spring, the leaves grow in slowly and allow partial sunlight when the home needs it.  In summer, leaves fully cover the arbor and extend it out to the full sheltering angle required to block the energy (transpiration adds additional cooling and makes for a pleasant sitting spot).  In Autumn, the leaves stay until the sunlight is too weak for photosynthesis or the temperature is too cold for transpiration and then they fall off and expose the windows to more sunlight just when you need it.  The only hard part is waiting for the plants to grow ;), fabric over the arbor can help in the mean time.

The roof, itself, can prevent much of the Sun’s energy from entering the home.  Conventional “hot roof” construction does this by setting up a convective cooling loop in the roof cavity.  As the sun heats the roof and the air below it, it rises and exits from a vent at (or near) the peak, cooler fresh air is drawn in from below the eaves (soffit vent).  Since convection is carrying most of the heat up and out of the roof, less of it is able to enter the home.  Some homes boast lighter colored shingles that reflect some heat.  More expensive designs call for a “cool roof”, this is basically a roof under the hot roof to further separate your home from the convective cycle, they cost almost as much as two roofs.

Earth sheltered homes take advantage of the earths heat storage capacity.  The mass of earth, cooled by the previous night, takes a lot of energy to warm up enough to overheat the home below.  Even more energy can be absorbed by water evaporating on the roof (latent heat of evaporation).  For thinly covered roofs, this process can lead to excessive drying and dead vegetation, but roofs with 2 or 3 ft of earth and typical rain fall are massive enough to retain sufficient water for native vegetation (overly drained roofs are another problem, the umbrella can help).

Deciduous trees growing near the home can also help because their summer leaves block the sun, but fall away to let the winter sunlight thru.   Transpiration from the leaves also has a dramatic cooling effect in summer (latent heat of evaporation absorbs a lot of energy).   Earth sheltered homes can take this to another level by having growing plants transpiring directly on the roof.

After allowing the energy to enter the home, it must be stored.  A balanced passive solar home, stick built on an insulated concrete slab, might have heat storage to make it thru a cold winters night on the previous days charge.  If it had more mass, more heat capacity, it could increase its glazed area and collect and store more energy.

A typical earth sheltered home already has plenty of mass due to the heavy structure required to support the roof.  If that mass is placed correctly, such a home can easily absorb several days worth of the solar energy without overheating.  This means the glass area must be increased to balance the storage capacity and keep the home comfortable.  If you extend this idea even further, a home with an insulation umbrella can store a seasons worth of energy.

The rate of heat transfer into the mass of the home is also important to consider.  Some authorities talk about keeping the more massive cement pad a dark color to aid with absorbing the heat of the sun.  If you have masonry walls, you can go with a lighter color for the floor, knowing that the reflected energy will just be absorbed by the walls (and not wasted overheating the space).  Many warn about not covering the floors with carpet or furniture.  Others talk about the top few inches being the most effective at storing the energy (because they expect to charge and use the energy on a daily cycle).

All other things being equal, a greater surface area allows for faster heat transfer.  In other words, the same amount of mass is more effective if it is spread out over a larger area.  A 4 or 6 inch thick concrete slab is a simple starting point.  Some ambitious passive solar home builders try to take this idea further by building “solar rock beds” and pass solar heated air thru the beds (either actively with fans or passively with natural convection).  The broken up rocks have a large amount of surface area and can effectively transfer heat as the air passes thru them.  I am sure these can work, if done carefully, but…  Rock bed heat storage systems represent a significant additional expense which does not help the structure in any other way, and a warm pile of potentially moist rocks is a potential breeding ground for mold and other problems.

In the other direction, it is equally important that the storage medium be able to return the heat to the home faster than it can be lost to the cold winter environment.   The steady state heat equation (Fourier’s law) is Q=A/R*(Δ T), This means that the energy transferred is proportional to the wall area and temperature difference, but inversely proportional to the R value of the wall.   Hopefully your buried wall/floor/ceiling area is greater than your window area, but it is probably not great enough to overcome the much greater temperature differential between the inside and outside.   The R value is what will help you balance out the equation so that you retain enough heat in the home to stay comfortable.  This means getting the R value of the windows as high as possible, as well as getting the R value of the buried walls as low as possible.   We can increase the R value of windows by insulating them at night.  This can be done with insulating curtains or shutters.  For the walls, remember that R value is cumulative thru all the “layers”, and can be reduced in more than one place.

… (more to come)